Adsorption of a linear polyelectrolyte on a gold electrode

Abstract

The adsorption of quaternized poly-2-vinyl pyridine (PVP+), which has a fixed charge per monomer, onto a gold electrode was investigated using reflectometry. The double layer charge and potential of the gold substrate were controlled by means of either the solution pH or by applying an external potential. The adsorption process was monitored for various molecular masses and concentrations of PVP+, and as a function of the electrolyte concentration, the pH of the solution, and the externally applied potential on the gold electrode. The adsorption isotherms and the dependence on the background electrolyte concentration point to a high non-electrostatic affinity of PVP+ for the gold surface. The results for the two charging mechanisms (pH or applied potential) are very similar when compared by plotting the adsorbed amounts as a function of the double layer potential of the gold. The total adsorbed amount decreases linearly with the double layer potential of the gold. Adsorption takes place up to a relatively high double layer potential, in line with a relatively high contribution of non-electrostatic interactions. The electrostatic barrier for adsorption is always low, since in practically all measurements the initial adsorption rate is completely determined by the transport of molecules towards the gold surface, even near the threshold potential for adsorption.